Bluetongue, Biology

Bluetongue

Bluetongue (BT) is a non-contagious and arboviral disease of both domestic and wild ruminants. The disease is enzootic in areas where reservoirs (cattle and wild ruminants) and vectors exist for the BT virus (BTV). The disease in sheep is characterized by stomatitis, rhinitis, enteritis and lameness.

Clinical signs: Initially, the affected animals show high rise of body tempera­ture followed by excessive salivation, nasal discharge and reddening of buccal and nasal mucous membranes. Later on saliva and nasal discharge becomes blood mixed and there is bad smell from mouth. Lips, gums, dental pad and tongue are swollen. Purple coloured necrotic lesions are noticed on the lateral aspect of tongue resulting in difficulty in swallowing. Laminitis, coronitis, lameness, occasional diarrhoea, breaking of wool and rapid loss of condition are also noticed and younger animals die within one week. However, recovered animals suffer from convulsions, abortions, and cracking of hooves and skin.

Affected cows usually show rise in body temperature, stiffness, laminitis affecting all the four limbs, excessive salivation, edema of lips, inappetance, nasal discharge, and ulceration on tongue, dental pad and muzzle, and congenital abnormalities.

Diagnosis: It is diagnosed by clinical sign and postmortem lesions in dead sheep. Blood examination reveals high activity of creatinine phosphokinase enzyme due to muscular degeneration. Disease can be confirmed by inoculating blood of suspected animal into unweaned white mice or hamster. Complement fixation or ELISA tests are also employed for its confirmatory diagnosis.

Treatment: There is no specific treatment for the disease but oral lesions can be washed with mild antiseptic solutions like 3% alum or weak KMnO4 solution. Lesions can be painted by 2% gentian violet to check secondary bacterial infection for which broad-spectrum antibiotics like streptopenicilin, tetracycline, erythromycin, ciprofloxacin or gentamycin may also be used.

Control:Adoption of strict hygienic and proper vector control measures can reduce the occurrence of disease. Prophylactic immunization of sheep against BT is the most practical and effective control measure to combat BT infection. At present, attenuated vaccines are used in the Republic of South Africa, the USA and other countries. However, EU countries were using attenuated vaccines, only recently shifting to inactivated vaccines owing to their safety and efficacy. In India, inactivated vaccines are in experimental stages and are expected to be on the market shortly. Inactivated vaccines generate serotype-specific long-lasting protective immunity after two injections, and may help in controlling epidemics. Attenuated live vaccines are efficacious but safety issues are of great concern. Recombinant vaccines, which generate cross-protection against multiple BTV serotypes, have great potential in BT vaccine regimens.